Support updating template processors

[ArthurZucker]

Goal:

from tokenizers import Tokenizer
from tokenizers.processors import TemplateProcessing
tokenizer = Tokenizer.from_pretrained("meta-llama/Llama-3.3-70B-Instruct")
tokenizer.post_processor 

tokenizer.post_processor[1] = TemplateProcessing(
    single="[CLS] $0 [SEP]",
    pair="[CLS] $A [SEP] $B:1 [SEP]:1",
    special_tokens=[("[CLS]", 1), ("[SEP]", 0)],
)

[HuggingFaceDocBuilderDev]

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